There is an increasing need for reliable, efficient and economical means for interconnecting conductive areas on two substrates. The anisotropically conductive adhesive disclosed herein provides one such means.
The use of conductive adhesive is well known. The problem with most conductive adhesive is that they conduct equally well in all directions. They, therefore, must be applied only to those areas where electrical conduction is desired.
An anisotropically conductive adhesive, on the other hand, is an adhesive composition which will permit electrical conductivity normal to the plane of the substrate to which the adhesive has been applied, while precluding conductivity between adjacent conductive areas in the plane of the substrate.
U.S. Pat. No. 4,170,677 discloses one such anisotropically conductive adhesive composition. This disclosure reveals the use of whisker-like particles which are either paramagnetic or ferromagnetic. Once the adhesive is coated on a substrate, the substrate must be exposed to a magnetic or electrical field to make the conductive whiskers align perpendicular to the substrate. The adhesive must then be hardened while the field is still applied.
U.S. Pat. No. 4,252,391 discloses a method for preparing anisotropically pressure-sensitive electroconductive composite sheets. These sheets are comprised of electrically conductive fibers that have been distributed in a matrix of an electrically insulating substance. The fibers are aligned in a direction substantially perpendicular to the plane of the sheet.
The anisotropically conductive adhesive as disclosed in the present invention eliminates the necessity for using an externally applied force for orienting or aligning any particles. Furthermore, the adhesive can be applied across the whole surface of the substrate. There is no need to apply the adhesive only to the conductive areas, nor to provide separate insulating means to preclude conductivity between adjacent areas in the same substrate.
An anisotropically conductive adhesive composition for use in electrically connecting at least one conductive area on one substrate with at least one conductive area on a second substrate, is disclosed herein. The adhesive composition is comprised of a nonhomogenous mixture of conductive particles in a nonconductive adhesive binder. The conductive particles, not requiring a specific shape, are dispersed randomly throughout the mixture in noncontiguous conductive units, each unit being comprised of individual particles. The units are sufficiently spaced apart to preclude electrical conductivity between adjacent conductive areas on the same substrate. Whereby, upon applying a layer of the adhesive composition over the conductive and insulating areas on the first substrate and positioning in a conducting relationship and adhering said at least one conductive area on the second substrate with said at least one area on the first substrate, the units establish electrical connection between the adhered conductive areas on the two substrates.
A method for electrically connecting at least one conductive area on one substrate with at least one conductive area on a second substrate is also disclosed. The method is comprised of the steps of: applying a layer of the herein disclosed anisotropically conductive composition over the conductive and insulating areas on a first substrate; positioning in a conducting relationship said at least one conductive area on the first substrate with said at least one conductive area on the second substrate; adhering said at least one conductive area on the first substrate with said at least one conductive area on the second substrate, whereby the noncontiguous conductive units establish electrical connection between the adhered conductive areas on the two substrates.
A product comprised of at least two substrates, each substrate having at least one conductive area and at least one insulating area on at least one surface thereof, said at least one conductive area on a first substrate being electrically connected to a corresponding conductive area on a second substrate is also disclosed. The electrical connection is accomplished by the method as taught herein.
The use of the anisotropically conductive adhesive composition can be understood by referring to the following drawings.